Messages in this thread | | | Subject | Re: [PATCH 0/4] arch, mm: improve robustness of direct map manipulation | From | David Hildenbrand <> | Date | Tue, 27 Oct 2020 11:34:49 +0100 |
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On 27.10.20 10:47, Mike Rapoport wrote: > On Tue, Oct 27, 2020 at 09:46:35AM +0100, David Hildenbrand wrote: >> On 27.10.20 09:38, Mike Rapoport wrote: >>> On Mon, Oct 26, 2020 at 06:05:30PM +0000, Edgecombe, Rick P wrote: >>>> On Mon, 2020-10-26 at 11:05 +0200, Mike Rapoport wrote: >>>>> On Mon, Oct 26, 2020 at 01:13:52AM +0000, Edgecombe, Rick P wrote: >>>>>> On Sun, 2020-10-25 at 12:15 +0200, Mike Rapoport wrote: >>>>>>> Indeed, for architectures that define >>>>>>> CONFIG_ARCH_HAS_SET_DIRECT_MAP >>>>>>> it is >>>>>>> possible that __kernel_map_pages() would fail, but since this >>>>>>> function is >>>>>>> void, the failure will go unnoticed. >>>>>> >>>>>> Could you elaborate on how this could happen? Do you mean during >>>>>> runtime today or if something new was introduced? >>>>> >>>>> A failure in__kernel_map_pages() may happen today. For instance, on >>>>> x86 >>>>> if the kernel is built with DEBUG_PAGEALLOC. >>>>> >>>>> __kernel_map_pages(page, 1, 0); >>>>> >>>>> will need to split, say, 2M page and during the split an allocation >>>>> of >>>>> page table could fail. >>>> >>>> On x86 at least, DEBUG_PAGEALLOC expects to never have to break a page >>>> on the direct map and even disables locking in cpa because it assumes >>>> this. If this is happening somehow anyway then we should probably fix >>>> that. Even if it's a debug feature, it will not be as useful if it is >>>> causing its own crashes. >>>> >>>> I'm still wondering if there is something I'm missing here. It seems >>>> like you are saying there is a bug in some arch's, so let's add a WARN >>>> in cross-arch code to log it as it crashes. A warn and making things >>>> clearer seem like good ideas, but if there is a bug we should fix it. >>>> The code around the callers still functionally assume re-mapping can't >>>> fail. >>> >>> Oh, I've meant x86 kernel *without* DEBUG_PAGEALLOC, and indeed the call >>> that unmaps pages back in safe_copy_page will just reset a 4K page to >>> NP because whatever made it NP at the first place already did the split. >>> >>> Still, on arm64 with DEBUG_PAGEALLOC=n there is a possibility of a race >>> between map/unmap dance in __vunmap() and safe_copy_page() that may >>> cause access to unmapped memory: >>> >>> __vunmap() >>> vm_remove_mappings() >>> set_direct_map_invalid() >>> safe_copy_page() >>> __kernel_map_pages() >>> return >>> do_copy_page() -> fault >>> >>> This is a theoretical bug, but it is still not nice :) >>> >>>>> Currently, the only user of __kernel_map_pages() outside >>>>> DEBUG_PAGEALLOC >>>>> is hibernation, but I think it would be safer to entirely prevent >>>>> usage >>>>> of __kernel_map_pages() when DEBUG_PAGEALLOC=n. >>>> >>>> I totally agree it's error prone FWIW. On x86, my mental model of how >>>> it is supposed to work is: If a page is 4k and NP it cannot fail to be >>>> remapped. set_direct_map_invalid_noflush() should result in 4k NP >>>> pages, and DEBUG_PAGEALLOC should result in all 4k pages on the direct >>>> map. Are you seeing this violated or do I have wrong assumptions? >>> >>> You are right, there is a set of assumptions about the remapping of the >>> direct map pages that make it all work, at least on x86. >>> But this is very subtle and it's not easy to wrap one's head around >>> this. >>> >>> That's why putting __kernel_map_pages() out of "common" use and >>> keep it only for DEBUG_PAGEALLOC would make things clearer. >>> >>>> Beyond whatever you are seeing, for the latter case of new things >>>> getting introduced to an interface with hidden dependencies... Another >>>> edge case could be a new caller to set_memory_np() could result in >>>> large NP pages. None of the callers today should cause this AFAICT, but >>>> it's not great to rely on the callers to know these details. >>> A caller of set_memory_*() or set_direct_map_*() should expect a failure >>> and be ready for that. So adding a WARN to safe_copy_page() is the first >>> step in that direction :) >>> >> >> I am probably missing something important, but why are we saving/restoring >> the content of pages that were explicitly removed from the identity mapping >> such that nobody will access them? >> >> Pages that are not allocated should contain garbage or be zero >> (init_on_free). That should be easy to handle without ever reading the page >> content. > > I'm not familiar with hibernation to say anything smart here, but the > help text of DEBUG_PAGEALLOC in Kconfig says: > > ... this option cannot be enabled in combination with > hibernation as that would result in incorrect warnings of memory > corruption after a resume because free pages are not saved to > the suspend image. > > Probably you are right and free pages need to be handled differently, > but it does not seem the case now. > >> The other user seems to be vm_remove_mappings(), where we only *temporarily* >> remove the mapping - while hibernating, that code shouldn't be active >> anymore I guess - or we could protect it from happening. > > Hmm, I _think_ vm_remove_mappings() shouldn't be active while > hibernating, but I'm not 100% sure. > >> As I expressed in another mail, secretmem pages should rather not be saved >> when hibernating - hibernation should be rather be disabled. > > Agree. > >> What am I missing? > > I think I miscommunicated the purpose of this set, which was to hide > __kernel_map_pages() under DEBUG_PAGEALLOC and make hibernation use > set_direct_map_*() explictly without major rework of free pages handling > during hibernation. > > Does it help? >
Heh, as always, once you touch questionable code, people will beg for proper cleanups instead :)
-- Thanks,
David / dhildenb
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